Bus ID's can be found by looking for 3 numbers (black 3-number decal) on the bus outside, or front of the bus above the windshield inside. sometimes there is also one on the back of the bus inside.

or if your a nerd (like me) go to Navigo select a bus stop, and then click an upcoming bus. Open dev tools and go to the network tab, then click "Trip Schedule" on navigo for your selected bus.
Look for the request to "./schedule?trip_id=", click it, and go to the response tab, and look for the "busId" key in the JSON. That is the busID for that bus.
(for some reason WPG Transit doesnt show you this on the page itself.)